Magnetic tunnel junction (MTJ) elements serving as magnetoresistive elements have a multilayer structure including a storage layer, in which the magnetization direction is variable, a reference layer, in which the magnetization direction is unchangeable, and an insulating layer disposed between the storage layer and the reference layer. The MTJ elements are known to have a tunneling magnetoresistive (TMR) effect, and used as storage elements of memory cells in magnetic random access memories (MRAMs).
MRAMs store data (“1”, “0”) based on changes in relative angle between magnetization directions of magnetic layers included in each MTJ element, and are nonvolatile memories. Since the magnetization may be switched in several nanoseconds, data may be written and read at a high speed. Therefore, the MRAMs are highly expected as next-generation high-speed nonvolatile memories. The cell size of the MRAMs may be reduced by employing spin transfer torque magnetization switching, in which the magnetizations are controlled by means of spin polarized currents. The reduction in cell size may lead to an increase in the current density. The increased current density may allow magnetization switching in storage layers to be performed more easily. Therefore, MRAMs with high density and low power consumption may be obtained.
In order to improve the density of nonvolatile memories, the magnetoresistive elements need be highly integrated. However, thermal stability of ferromagnetic materials, which form the magnetoresistive elements, may be degraded if the entire device size is reduced. Therefore, improvement in the magnetic anisotropy and the thermal stability of the ferromagnetic materials is a problem.
In order to solve this problem, attempts have recently been made to produce MRAMs including perpendicular magnetization MTJ elements, in which the magnetizations of the ferromagnetic materials are perpendicular to the film plane. The magnetic materials to form perpendicular magnetization MTJ elements need to have perpendicular magnetic anisotropy. In order to achieve the perpendicular magnetic anisotropy, materials having crystalline magnetic anisotropy or interface magnetic anisotropy are selected. For example, FePt, CoPt, and FePd have strong crystalline magnetic anisotropy. A number of MTJ elements including an MgO tunnel barrier layer and a layer with interface perpendicular magnetic anisotropy, such as a layer of CoFeB, are reported.
An MTJ element includes a storage layer, a reference layer, and a tunnel barrier layer disposed between the storage layer and the reference layer. The storage layer and the reference layer contain magnetic material, and emit a magnetic field to the outside. In a common MTJ element including perpendicular-magnetization storage layer and reference layer, the stray magnetic field from the reference layer is greater than that from a reference layer included in an in-plane magnetization type MTJ element, in which the magnetization of a ferromagnetic material is parallel to the film plane. The storage layer, which has a smaller coercive force than the reference layer, of the perpendicular magnetization type MTJ element is strongly influenced by the stray magnetic field from the reference layer. Specifically, problems such as a shift in magnetic field for magnetization switching, and a degradation in thermal stability of the storage layer may be caused due to the influence of the stray magnetic field from the reference layer.
Countermeasures proposed to reduce the stray magnetic field from the reference layer to the storage layer in a perpendicular magnetization MTJ element include a reduction in saturation magnetization in the reference layer, and an addition of a magnetic layer (shift adjustment layer) with a magnetization that is directed to cancel the magnetization of the reference layer. The reduction in saturation magnetization of the reference layer, however, may lead to degradation in thermal stability of the reference layer itself. The degradation in thermal stability may lead to the switching of the direction of magnetization of the reference layer due to the stray magnetic field from the shift adjustment layer or the storage layer while a temperature is increased during, for example, reflow soldering of the magnetic memory array. As a result, the magnetization direction of the reference layer and the magnetization direction of the shift adjustment layer, which need to act to cancel the stray magnetic fields from these layers, match to apply a greater stray magnetic field to the storage layer.